When Hans Selye was experimenting on rats by inflicting stress them either by injecting them with hormones or chemicals, making surgical incisions or exposing them to extreme temperatures, he noticed that the rats were all displaying the same group of symptoms. At first he believed that he had discovered a new hormone1; however, several years of further testing by injecting the rats with other substances, such as formaldehyde, revealed the same results. Even exposure to cold, cutting their spinal cords and forced exercise produced the same effects. The effects occurred in a predictable sequence that is now known as the General Adaptation Syndrome2 (GAS).

Stage 1 of the General Adaptation Syndrome: Alarm

Selye called the first stage of the General Adaptation Syndrome the General Alarm Reaction. This reaction is the result of the biological processes that make up the body's response to stress stress3 Signals are sent to the sympathetic division of the ANS, which increases heart rate, dilates bronchi, inhibits digestion, converts glycogen to glucose etc; meanwhile the hypothalamus also releases CRF, which in turn prompts the release of ACTH from the pituitary which stimulates the secretion of cortisol4 from the adrenal-cortex.

Within six to forty-eight hours of the injury, the rats displayed a rapid decrease in the size of the Thymus, Spleen, Lymph Glands and Liver; loss of fat; Oedema formation; loss of muscle tone; fall of body temperature; formation of acute erosions in the digestive tract, particularly in the stomach, small intestine, and appendix; loss of cortical lipoids and adrenaline secreting cells from the adrenal medulla; sometimes increased blood flow to the skin; exophthalmos; and increased lachrymation and salivation; in particularly severe cases there was focal necrosis of the liver and dense clouding of the crystalline lens.

Stage 2 of the General Adaptation Syndrome: Resistance

If the stressor is not removed the body attempts to adapt to the stress and in the second stage, resistance, it is largely successful; nevertheless, adrenaline and cortisol levels remain high. The consequence of this is that although the body is able to cope, its resources are depleted. If the stress continues for longer the third stage of the syndrome is reached.

About 48 hours after the injury Selye found that the adrenal glands were still enlarged, but there had been some recovery of the cortical lipoids, there was a large number of hormone producing cells in the pituitary, the thyroid gland had grown by producing new cells; however, body growth stopped and the gonads had become atrophic.

Selye prolonged the animals' stress by giving repeated doses of the drug over a period of 1-3 months. He found that the rats were able to build up a resistance to the stress and the organs appeared to return to normal.

Stage 3 of the General Adaptation Syndrome: Exhaustion

Resistance to stress can only be maintained for a limited amount of time. As the body's resources become depleted, the immune system is weakened and health problems develop.

Selye found that after the resistance period, the rats were unable to maintain their stress defences. They began to develop symptoms that were similar to those in the first stage. Selye considered this to be due to exhaustion.

The results of these experiments led Selye to propose the General Adaptation Syndrome.

The above is an adaptation of two separate articles by Keiron Walsh: "The General Adaptation Syndrome and "Han Selye's Experiments on Rats"

1. Hormones are molecules that act as chemical messengers. They are released by one part of the body and have an affect on another part of the body. They are often transported through the bloodstream.

2. In medicine and psychology the term syndrome refers to the association of several clinically recognizable features, signs (discovered by a physician), symptoms (reported by the patient), phenomena or characteristics which often occur together so that the presence of one feature alerts the physician to the presence of the others (source wikipedia

3. . Stress occurs when a person perceives there to be a mismatch between the demands of a situation and their ability to cope with these demands.

4. Cortisol is a hormone released by the adrenal cortex. It increases blood pressure and the amount of sugar available in the bloodstream. It is also an immunosuppressant, which means it inhibits immune response. Cortisol is often referred to as the stress hormone as its production is triggered by the stress response.